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Using and Porting GNU Fortran

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8.1 Direction of Language Development

The purpose of the following description of the GNU Fortran language is to promote wide portability of GNU Fortran programs.

GNU Fortran is an evolving language, due to the fact that g77 itself is in beta test. Some current features of the language might later be redefined as dialects of Fortran supported by g77 when better ways to express these features are added to g77, for example. Such features would still be supported by g77, but would be available only when one or more command-line options were used.

The GNU Fortran language is distinct from the GNU Fortran compilation system (g77).

For example, g77 supports various dialects of Fortran--in a sense, these are languages other than GNU Fortran--though its primary purpose is to support the GNU Fortran language, which also is described in its documentation and by its implementation.

On the other hand, non-GNU compilers might offer support for the GNU Fortran language, and are encouraged to do so.

Currently, the GNU Fortran language is a fairly fuzzy object. It represents something of a cross between what g77 accepts when compiling using the prevailing defaults and what this document describes as being part of the language.

Future versions of g77 are expected to clarify the definition of the language in the documentation. Often, this will mean adding new features to the language, in the form of both new documentation and new support in g77. However, it might occasionally mean removing a feature from the language itself to "dialect" status. In such a case, the documentation would be adjusted to reflect the change, and g77 itself would likely be changed to require one or more command-line options to continue supporting the feature.

The development of the GNU Fortran language is intended to strike a balance between:

One of the biggest practical challenges for the developers of the GNU Fortran language is meeting the sometimes contradictory demands of the above items.

For example, a feature might be widely used in one popular environment, but the exact same code that utilizes that feature might not work as expected--perhaps it might mean something entirely different--in another popular environment.

Traditionally, Fortran compilers--even portable ones--have solved this problem by simply offering the appropriate feature to users of the respective systems. This approach treats users of various Fortran systems and dialects as remote "islands", or camps, of programmers, and assume that these camps rarely come into contact with each other (or, especially, with each other's code).

Project GNU takes a radically different approach to software and language design, in that it assumes that users of GNU software do not necessarily care what kind of underlying system they are using, regardless of whether they are using software (at the user-interface level) or writing it (for example, writing Fortran or C code).

As such, GNU users rarely need consider just what kind of underlying hardware (or, in many cases, operating system) they are using at any particular time. They can use and write software designed for a general-purpose, widely portable, heterogenous environment--the GNU environment.

In line with this philosophy, GNU Fortran must evolve into a product that is widely ported and portable not only in the sense that it can be successfully built, installed, and run by users, but in the larger sense that its users can use it in the same way, and expect largely the same behaviors from it, regardless of the kind of system they are using at any particular time.

This approach constrains the solutions g77 can use to resolve conflicts between various camps of Fortran users. If these two camps disagree about what a particular construct should mean, g77 cannot simply be changed to treat that particular construct as having one meaning without comment (such as a warning), lest the users expecting it to have the other meaning are unpleasantly surprised that their code misbehaves when executed.

The use of the ASCII backslash character in character constants is an excellent (and still somewhat unresolved) example of this kind of controversy. See section 15.5.1 Backslash in Constants. Other examples are likely to arise in the future, as g77 developers strive to improve its ability to accept an ever-wider variety of existing Fortran code without requiring significant modifications to said code.

Development of GNU Fortran is further constrained by the desire to avoid requiring programmers to change their code. This is important because it allows programmers, administrators, and others to more faithfully evaluate and validate g77 (as an overall product and as new versions are distributed) without having to support multiple versions of their programs so that they continue to work the same way on their existing systems (non-GNU perhaps, but possibly also earlier versions of g77).


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